Fast Determination of Iodine Number of Biodiesel Using Capillary Zone Electrophoresis with Multi- and Single-Point Calibration

A new and fast method was developed to determine the iodine number of biodiesel samples by capillary zone electrophoresis (CZE). In this procedure, the biodiesel sample reacts with Wijs solution in the presence of a catalyst and all the remaining iodine monochloride is converted to the iodide ion, which is quantified by CZE. Only small amounts of sample and reactant are required and µL level amounts of chloroform and Wijs solution. The reaction time was reduced to 3 min and the total analysis time is less than 15 min. The simplicity and speed of the method can be further enhanced by using single-point calibration. The limits of quantification (LOQ) and determination (LOD) values were better than 0.6 and 0.18 g per 100 g of biodiesel. Instrumental, intraday, and interday precisions were better than 1.13, 1.04 and 6.02% relative standard deviation (RSD). The accuracy was statistically verified by comparison with the standard method EN 14111.

Keywords:
iodine number; biodiesel; capillary zone electrophoresis; Wijs

Authorship

Alysson V. F. Sako

Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis-SC, BrazilUniversidade Federal de Santa CatarinaBrazilFlorianópolis, SC, BrazilDepartamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis-SC, Brazil

Gustavo A. Micke ^**e-mail: gustavo.micke@ufsc.br

https://orcid.org/0000-0002-5518-624X

*e-mail: gustavo.micke@ufsc.br

SCIMAGO INSTITUTIONS RANKINGS

Figures | Tables | Formulas

Figures (2)
Tables (3)
Formulas (5)

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Figure 1
Simplified model for the proposed method. Electropherogram of biodiesel sample A observed at 230 nm. CE configuration: fused-silica capillary of 48.5 cm × 50 µm i.d.; hydrodynamic injection: 50 mbar for 5 s; separation voltage: –30 kV; cartridge temperature: 25 ºC; BGE composition: 50 mmol L^-1 6-aminocaproic acid adjusted to pH 5.02 with HCl. Detailed information of the method is described in the Experimental section.

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Figure 2
Comparison between MPC and SPC using t-test with the response factors for each i level (R_{f_i}). Between the accepted t-zone, R_f is approximately equal to R_f and SPC is statistically equivalent to the MPC results for biodiesel sample quantifications.

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Table 1
Half-reactions, standard reduction potentials and global standard reaction for the proposed direct reaction between ICl and S₂O₃^2–

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Table 2
Validation parameters for MPC and SPC methods

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Table 3
IN found using the proposed method for five different biodiesel samples calculated using MPC and SPC methods

(1)

(2)

(3)

(4)

(5)

Figure 1 Simplified model for the proposed method. Electropherogram of biodiesel sample A observed at 230 nm. CE configuration: fused-silica capillary of 48.5 cm × 50 µm i.d.; hydrodynamic injection: 50 mbar for 5 s; separation voltage: –30 kV; cartridge temperature: 25 ºC; BGE composition: 50 mmol L^-1 6-aminocaproic acid adjusted to pH 5.02 with HCl. Detailed information of the method is described in the Experimental section.

Figure 2 Comparison between MPC and SPC using t-test with the response factors for each i level (R_{f_i}). Between the accepted t-zone, R_f is approximately equal to R_f and SPC is statistically equivalent to the MPC results for biodiesel sample quantifications.

Table 1 Half-reactions, standard reduction potentials and global standard reaction for the proposed direct reaction between ICl and S₂O₃^2–

Semi-reaction^a a Data adapted from Harris.27 E0red: standard reduction potential; E0global red: global standard reduction potential.	Standard reduction potential^a a Data adapted from Harris.27 E0red: standard reduction potential; E0global red: global standard reduction potential. / V
$2 IC 1_{(s)} + 2 e^{-} ⇌ I_{2} + 2 C 1^{-}$	E⁰_red = +1.22
$S_{2} O_{3}^{2 -} + 3 H_{2} O ⇌ 2 H_{2} {SO}_{3} + 3 H^{+} + 4 e^{-}$	E⁰_red = –0.40
$I_{2 (aq)} + 2 e^{-} ⇌ 2 I^{-}$	E⁰_red = +0.62
Global reaction	Global standard potential / V
$\begin{matrix} 2 IC 1_{(s)} + S_{2} O_{3}^{2 -} + 3 H_{2} O ⇌ 2 H_{2} {SO}_{3} + \\ 3 H^{+} + 2 I^{-} + 2 C 1^{-} \end{matrix}$	E⁰_{global red} = +1.44

^a
Data adapted from Harris.²⁷ E⁰_red: standard reduction potential; E⁰_{global red}: global standard reduction potential.

Table 2 Validation parameters for MPC and SPC methods

Parameter	Multi-point calibration (MPC)	Single-point calibration (SPC)
Linear regression equation	${Ar}_{1^{-}} / {Ar}_{{NO}_{3}^{-}} = 0.0125 C_{1^{-}} + 0.0062$	${Ar}_{1^{-}} / {Ar}_{{NO}_{3}^{-}} = 0.6239 C_{1^{-}} / C_{{NO}_{3}^{-}}$
Linear range / (mg L^-1)	5.0-90.0
Linearity	$F_{calculated} = 0.296 < [F_{(0.05, 5, 14)} = F_{critical} = 2.958]; r^{2} = 0.9998$
Instrumental precision / (% RSD)	1.13
Intraday precision / (% RSD)	0.036-1.04
Interday precision / (% RSD)	1.50-6.02
LOQ^b b considering the sample mass and dilution in the “Sample preparation procedure and Wijs reagent quantification” sub-section. Based on S/N ratio of 10:1 for MPC, and based on equations 2 and 4 for SPC. If converted to ICl values, limits of quantification (LOQs) are 0.38 and 0.14 g per 100 g of biodiesel for MPC and SPC, respectively; / (g I₂per 100g of biodiesel)	0.60	0.165
LOD^c c considering the sample mass and dilution in the “Sample preparation procedure and Wijs reagent quantification” sub-section. Based on S/N ratio of 3:1 for external calibration curve, and based on equations 3 and 4 for SPC. If converted to ICl values, limits of detection (LODs) are 0.12 and 0.05 g per 100 g of biodiesel for MPC and SPC, respectively; / (g I₂per 100g of biodiesel)	0.18	0.055
Accuracy^d d reference sample A was analyzed by 12 laboratories and reference sample E by 1 laboratory using the DIN EN 1411116 standard method. Fcalculated and Fcritical: calculated and tabulated (Fisher-Snedecor distribution) F-test values; r2: coefficient of determination; RSD: relative standard deviation; tcalculated and tcritical: calculated and tabulated Student’s t-values.	Accurate with 95% of confidence (t-test)
	reference sample A: t_calculated = −0.87 (t_critical = 2.16) reference sample E: t_calculated = 2.70 (t_critical = 2.78)	reference sample A: t_calculated = −1.72 (t_critical = 2.16) reference sample E: t_calculated = 2.39 (t_critical = 2.78)

^a
Since –t_{α / 2,n – 2} < b0 / s0 < +t_{α / 2,n – 2} (where α is the confidence level; b₀ is the intercept from the linear regression; and s₀ is the standard error of the intercept from the linear regression), the null hypothesis (intercept = 0) is accepted and single-point calibration is possible;³⁴34 Bánfai, B.; Kemény, S.; J. Chemom. 2012, 26, 117.
^b
considering the sample mass and dilution in the “Sample preparation procedure and Wijs reagent quantification” sub-section. Based on S/N ratio of 10:1 for MPC, and based on equations 2 and 4 for SPC. If converted to ICl values, limits of quantification (LOQs) are 0.38 and 0.14 g per 100 g of biodiesel for MPC and SPC, respectively;
^c
considering the sample mass and dilution in the “Sample preparation procedure and Wijs reagent quantification” sub-section. Based on S/N ratio of 3:1 for external calibration curve, and based on equations 3 and 4 for SPC. If converted to ICl values, limits of detection (LODs) are 0.12 and 0.05 g per 100 g of biodiesel for MPC and SPC, respectively;
^d
reference sample A was analyzed by 12 laboratories and reference sample E by 1 laboratory using the DIN EN 14111¹⁶ standard method. F_calculated and F_critical: calculated and tabulated (Fisher-Snedecor distribution) F-test values; r²: coefficient of determination; RSD: relative standard deviation; t_calculated and t_critical: calculated and tabulated Student’s t-values.

Table 3 IN found using the proposed method for five different biodiesel samples calculated using MPC and SPC methods

Biodiesel sample	Iodine number / (g I₂per 100 g of sample)
Biodiesel sample	Multi-point calibration (MPC)	Single-point calibration (SPC)
A^a a Reference method: DIN EN 14111,16 quantified by 12 laboratories. Reference value: 130 ± 2 g I2 per 100 g of sample;	132 ± 1	131 ± 1
B	126 ± 3	126 ± 3
C	128 ± 5	128 ± 5
D	124 ± 5	124 ± 5
E^b b reference method: DIN EN 14111,16 quantified by 1 laboratory. Reference value: 62 ± 1 g I2 per 100 g of sample.	58 ± 3	57 ± 3

^a
Reference method: DIN EN 14111,¹⁶16 En 14111: Fat and Oil Derivatives - Fatty Acid Methyl Esters (FAME), Determination of Iodine Value, European Committee for Standardization, Berlin, 2003. quantified by 12 laboratories. Reference value: 130 ± 2 g I₂ per 100 g of sample;
^b
reference method: DIN EN 14111,¹⁶16 En 14111: Fat and Oil Derivatives - Fatty Acid Methyl Esters (FAME), Determination of Iodine Value, European Committee for Standardization, Berlin, 2003. quantified by 1 laboratory. Reference value: 62 ± 1 g I₂ per 100 g of sample.

(1)

F_{calculated} = \frac{Σ_{i = 1}^{p} m_{i} {({\overset{─}{y}}_{i} - {\hat{y}}_{i})}^{2} / (p - 2)}{Σ_{i = 1}^{p} Σ_{j = h}^{m_{i}} {(y_{ij} - {\overset{─}{y}}_{i})}^{2} / (m - p)}

(2)

\frac{\overset{─}{{Ar}_{i}}}{{\overset{─}{{Ar}_{i}}}_{{NO}_{3}^{-}}} = R_{f} \frac{C_{i}}{C_{{iNO}_{3}^{-}}}

(3)

LOD = \frac{3.3 S_{y / x}}{A} \sqrt{1 + h_{0} + 1 / I}

(4)

LOQ = \frac{10 S_{y / x}}{A} \sqrt{1 + h_{0} + 1 / I}

(5)

h_{0} = \frac{{\overset{─}{C}}_{cal}^{2}}{Σ_{i = 1}^{I} {(C_{i} - {\overset{─}{C}}_{cal})}^{2}}

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Fast Determination of Iodine Number of Biodiesel Using Capillary Zone Electrophoresis with Multi- and Single-Point Calibration

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[1] *e-mail: gustavo.micke@ufsc.br